Unroll the thawed puff pastry on a lightly floured counter. Brush the surface with half of the melted butter (15 g), then roll it up tightly into a log about 25 cm / 10 inches long. Wrap the log and chill it for 20 minutes while you make the custard.

Combine the sugar, water, cinnamon stick, and lemon peel in the 1-quart / 950 ml saucepan. Bring to a boil over medium heat, then simmer for 5 minutes without stirring, until slightly syrupy. Take the pan off the heat.

Whisk the 30 g flour with 60 ml of the cold milk in a bowl until no lumps remain. Heat the remaining 240 ml milk, the cream, and the salt in the 2-quart / 1.9 L saucepan over medium heat for 3 to 4 minutes, until steaming at the edges. Whisk in the flour mixture and cook for 3 to 4 minutes, stirring constantly, until it thickens like pudding. Lift the cinnamon and lemon peel out of the syrup, then pour the hot syrup into the milk base in a slow stream while whisking. Cool for 10 minutes, stirring now and then, until warm to the touch.

Whisk the egg yolks and vanilla into the cooled custard until fully blended. Strain through the fine-mesh sieve into the jug.

Set a rack in the upper third of the oven and heat to 250 C / 480 F. Brush the 12-cup muffin tin with the remaining melted butter. Unwrap the pastry log and slice it into 12 discs about 2 cm / 3/4 inch thick. Place each disc cut side up in a cup, dip your thumbs in cold water, and press into the center, then push the pastry up the sides until the walls are about 3 mm thick and just clear the rim.

Fill each shell about three-quarters full, roughly 60 ml. Bake for 13 to 15 minutes, checking from 12 minutes, until the pastry edges are deep golden and the custard tops have dark brown to black blisters.

Cool the tarts in the tin for 10 minutes, then loosen each with a butter knife and move to a wire rack. Dust with ground cinnamon and powdered sugar if you like, and serve warm.